Combination of a Zeolite-Based SCR Catalyst with a Manganese-Based SCR Catalyst in the Bypass

ABSTRACT

The invention relates to an exhaust-gas aftertreatment system for selective catalytic reduction with a plurality of SCR catalytic converters, which exhaust-gas aftertreatment system is able to reduce NOx in a large temperature range and can store SOx. The invention further relates to a method for treating an exhaust gas flow, in which method the exhaust-gas aftertreatment system according to the invention is used. The system comprises a high-temperature SCR catalyst for temperature ranges between 250° C. and 750° C. and a low-temperature SCR catalyst arranged downstream thereof for temperature ranges between 60° C. and less than 250° C. There is a reductant supply system directly upstream of the high-temperature SCR catalyst. The high-temperature SCR catalyst is designed to reduce NOx in exhaust gas that has a temperature above a temperature threshold value and to store SOx in the temperature range below the threshold value. The low-temperature SCR catalyst reduces NOx in the temperature range below the threshold value. In each case, the exhaust gas flows through the high-temperature SCR catalyst. An exhaust-gas bypass valve or flow control valve is arranged directly upstream of the low-temperature SCR catalyst. If the temperature of the exhaust gas is greater than or equal to the temperature threshold value, the exhaust gas is completely conducted past the low-temperature SCR catalyst. The high-temperature SCR catalyst advantageously contains a molecular sieve as a catalytically active layer, and the catalytically active layer of the low-temperature SCR catalyst is preferably a manganese-containing mixed oxide.

INTRODUCTION

The present invention relates to an exhaust gas aftertreatment system for selective catalytic reduction with a plurality of SCR catalysts, which system can both reduce NO_(x) in a large temperature range and store SO_(x). The present invention also relates to a method for treating an exhaust gas flow, in which method the exhaust gas aftertreatment system according to the invention is used.

PRIOR ART

The SCR systems known in the prior art comprise SCR catalysts which effectively reduce nitrogen oxides NO_(x) from exhaust gas flows of internal combustion engines during operation in normal to high temperature ranges, for example in temperature ranges between approximately 250° C. and 450° C. SCR in this case stands for “selective catalytic reduction.” However, during the cold start of an engine but also in low-load operation, the exhaust gas temperatures may fall to low temperature ranges between approximately 60° C. and approximately 250° C. In such temperature ranges, conventional SCR catalysts do not succeed in effectively reducing NO_(x) from exhaust gas flows. The reason for this is that engines generally generate only little NO₂ and oxidation catalysts do not convert enough NO into NO₂ at the temperatures during the cold start and the warm-up phase (<150° C.) for the NO_(x) reduction to become effective. Moreover, it is difficult to sufficiently provide ammonia from urea in such temperature range for effective NO_(x) reduction, since thermolysis and hydrolysis of the urea sometimes proceed incompletely. In order to convert more NO to NO₂ in low exhaust gas temperature operating ranges, an oxidation catalyst with large amounts of platinum can be used. Oxidation catalysts of this type are costly and can entail additional disadvantages. SCR catalysts which are particularly suitable for the normal to high temperature range are referred to below as HT-SCR.

The person skilled in the art is also aware of so-called low-temperature SCR catalysts, which can effectively reduce nitrogen oxides at low temperatures below a temperature threshold value of 100° C. to 250° C., sometimes even at temperatures below 100° C. Such low-temperature SCR catalysts are hereinafter referred to as TT-SCRs. These are combinations of individual or a mixture of transition metals or transition metal oxides, which are applied to oxides, mixed oxides or a combination of a plurality of oxides or mixed oxides. Particular TT-SCRs which contain manganese-containing mixed oxides or manganese or manganese oxide supported on metal oxides exhibit very high NO conversions even at low temperatures, sometimes even below 100° C. However, a disadvantage of such manganese-containing TT-SCRs is that activity and selectivity are very poor at high temperatures, so that, for example, large amounts of N₂O are formed. They also have a low stability to high aging temperatures and tend to be contaminated by SO_(x), wherein the possibilities for desulfurization are limited due to low aging stability.

DE 11 2011 003 613 T5 describes combined systems of HT-SCRs and TT-SCRs. If necessary, the exhaust gas can be conducted past the Mn-containing TT-SCR via a simple bypass system starting from a limit temperature T_(max). Although such systems may be suitable for circumventing the disadvantages of poor activity and selectivity at high temperatures as well as low stability to high temperatures, they only insufficiently solve the problem of sulfur contamination of the TT-SCR: As long as exhaust gas is conducted via the TT-SCR, sulfur contamination occurs. In such configurations, regeneration of the TT-SCR is possible only to a limited extent because of its low thermal stability. System arrangements in which the TT-SCR is arranged downstream of the HT-SCR can only inadequately utilize the outstanding activity of Mn-containing SCR catalysts at low temperatures, since the upstream HT-SCR catalysts store considerable amounts of reductant (NH₃). The reductant thus reaches the TT-SCR only after a considerable time delay, which represents a further disadvantage.

WO 2016/028290 A1 proposes avoiding sulfurization of the SCR catalyst by connecting a selective alkali metal- or alkaline earth metal-containing SO₃ trap upstream. This SO₃ trap can be combined with a second SCR catalyst in a suitable manner, for example through a layer or zone structure. If the capacity of the SO₃ trap is exhausted, replacement of the component or thermal regeneration takes place. The solution proposed in WO 2016/028290 A1 for avoiding sulfur contamination on an SCR catalyst is particularly unsuitable for manganese-containing catalysts. For this purpose, it is necessary to remove SO₂ from the exhaust gas in addition to SO₃. Thus, a routine replacement of the SO₃ trap would not be possible because of operating times that are too short. However, thermal regeneration would have the disadvantage that the SO₃ emitted from the SO₃ trap migrates to the downstream TT-SCR and the protection of such manganese-containing TT-SCR catalyst thus does not function permanently.

Furthermore, the solution proposed in WO 2016/028290 A1 is unsuitable for avoiding sulfur contamination on an SCR catalyst for systems in which an oxidation catalyst is connected upstream of the SCR catalyst. The amount of SO₃ emitted by the engine in the entire SO_(x) is comparatively small. The capacity of the SO₃ trap is thus sufficiently large before replacement or thermal regeneration must take place. However, if the SO_(x)-containing exhaust gas is first conducted over an oxidation catalyst, a large amount is oxidized to SO₃. As a result, the amount of SO₃ to be adsorbed increases considerably, so that replacement or thermal regeneration would have to take place significantly more frequently.

WO 2016/018778 A1 discloses exhaust gas aftertreatment systems comprising a TT-SCR and an HT-SCR. The exhaust gas aftertreatment system furthermore comprises a bypass: The exhaust gas is conducted over the TT-SCR at low temperatures, over the HT-SCR at higher temperatures. The TT-SCR contains a mixture of catalytically active metals, which are preferably applied to a beta zeolite. The mixture of catalytically active metals contains at least one mixture selected from Cu and Ce, Mn and Ce, Mn and Fe, Cu and W, and Ce and W, and at least one alkali metal and/or one metal from the group of lanthanides. The disclosure does not contain any information about the precise design of the other exhaust gas purification components, i.e., the DOC, the HT-SCR and the DPF. Here, DOC stands for “diesel oxidation catalyst” and DPF stands for “diesel particulate filter.” This document also does not contain any information about a possible impairment of the TT-SCR if the exhaust gas contains sulfur oxides SO_(x) and they are conducted over the TT-SCR.

Low-temperature SCR catalysts are described, for example, in Junhua Li, Huazhen Chang, Lei Ma, Jinming Hao and Ralph T. Yang: “Low-temperature selective catalytic reduction of NO_(x) with NH₃ over metal oxide and zeolite catalysts—A review,” Catal Today 2011, 175, 147-156. In this review, TT-SCRs with a single, manganese oxides-containing metal oxide catalyst, composite metal oxide catalysts based on mixtures of MnO₂ and other metal oxides, such as CeO₂ and Nb₂O₅, MnO₂-containing catalysts supported on metal oxides, such as Al₂O₃ or TiO₂, as well as Cu- or Fe-containing zeolites are compared with each other. TT-SCRs containing manganese oxides showed decreasing selectivity with respect to N₂ formation with increasing temperature of the exhaust gas. A major obstacle when using MnO₂-containing TT-SCRs is their low resistance to H₂O and SO₂. This poses a challenge for the application of manganese-containing SCRs at low temperatures. SO_(x) in the exhaust gas can lead to deactivation because of the occupation of the active centers of the catalyst by manganese or ammonium sulfates. Zeolites containing Fe and Cu are essentially more stable to SO₂ and exhibit good thermal stability, but have significantly lower NO_(x) conversion rates at low temperatures than manganese-containing catalysts. The reactivity of Fe-containing zeolites in the NO_(x) conversion is lower than that of Cu zeolites. On the other hand, the activity of Cu zeolites with respect to NO_(x) decreases markedly more in the presence of H₂O. This is likely to be explained by thermal degradation of the zeolite carrier and/or the formation of copper aluminate by dealumination, the decrease in the number of active reaction centers by conversion of Cu²⁺ into CuO, the redistribution of the reaction centers by the migration of Cu²⁺, or a combination of such mechanisms. Among iron-containing zeolites, Fe beta zeolites have a fairly good NO_(x) conversion rate at low temperatures. On the other hand, they react particularly sensitively to hydrocarbon sooting with incomplete combustion of the fuel in diesel engines.

Manganese oxide-based TT-SCR catalysts were also described in Chang Liu, Jian-Wen Shi, Chen Gao and Chunming Niu: “Manganese oxide-based catalysts for low temperature selective catalytic reduction of NO_(x) with NH₃: A review,” Appl Catal A 2016, 522, 54-69. The catalysts investigated were divided into four categories: simple MnO_(x) catalysts, manganese-based metal oxide mixtures, supported manganese-based metal oxide mixtures and manganese-based monolithic catalysts. Manganese-based metal oxide mixtures, for example of Fe and Mn oxides, were found to have the highest resistance to H₂O and SO₂. Nevertheless, for practical use, such systems are still not resistant enough to be able to operate them without additional precautions.

Isabella Nova, Enrico Tronconi (publisher): “Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts,” Fundamental and Applied Catalysis, Springer-Verlag 2014, Chapter 5, pp. 123-147, describes SCR catalysts based on Cu zeolites, Fe zeolites and vanadium for the NO_(x) exhaust gas purification in diesel vehicles. As V SCR catalysts, it is possible to use so-called mixed oxide catalysts, which are based on oxides of vanadium and which generally also contain oxides of titanium and further metals, such as tungsten.

To date, it has not been possible to provide an SCR system that is capable of effectively reducing nitrogen oxides from exhaust gases in low (60° C. to less than 250° C.), medium (250° C. to less than 450° C.) and high temperature ranges (450° C. to 700° C.), i.e., from approximately 60° C. to approximately 700° C. HT-SCRs work effectively only in medium to high temperature ranges, and TT-SCRs exhibit low activities and selectivities at high temperatures, are not stable at high aging temperatures, and are easily contaminated by SO_(x) in the exhaust gas flow. Therefore, there is a need for SCR systems that overcome the disadvantages of the prior art.

Object of the Invention

The object of the present invention is to provide an SCR system that effectively reduces nitrogen oxides from exhaust gases from internal combustion engines in low (60° C. to less than 250° C.), medium (250° C. to less than 450° C.) and high temperature ranges (450° C. to 700° C.), has high activity, selectivity and temperature stability, and is not contaminated by SO_(x) in the exhaust gas flow. A further object of the present invention is a method for treating an exhaust gas flow, comprising such an SCR system.

Achievement of the Object

The object of providing an SCR system which effectively reduces nitrogen oxides from exhaust gases of internal combustion engines in low, medium and high temperature ranges, has high activity, selectivity and temperature stability, and is not contaminated by SO_(x) in the exhaust gas flow is achieved according to the invention by an exhaust gas aftertreatment system which can be coupled to an internal combustion engine in such a way that it receives the exhaust gas flow, comprising

-   -   a selective catalytic reduction catalyst for medium to high         temperature ranges (HT-SCR), wherein the medium temperature         range comprises temperatures of 250° C. to less than 450° C. and         the high temperature range comprises temperatures of 450° C. to         750° C., which catalyst is designed both         -   to reduce NO_(x) in an exhaust gas having a temperature             above a temperature threshold value,             -   and         -   to store the SO_(x) contained in the exhaust gas across the             temperature range of the exhaust gas which is below a             temperature threshold value,     -   a reductant supply system arranged directly upstream of the         HT-SCR,     -   a low-temperature SCR (TT-SCR) arranged downstream of the         HT-SCR, wherein the low temperature range comprises temperatures         of 60° C. to less than 250° C., and wherein the TT-SCR is         designed to reduce NO_(x) in an exhaust gas having a temperature         below a temperature threshold value,     -   a temperature sensor arranged directly downstream of the HT-SCR         and measuring the temperature of the exhaust gas flow exiting         such HT-SCR,     -   an exhaust gas bypass and/or flow control valve designed to         conduct the exhaust gas flow in its entirety past the TT-SCR, if         such exhaust gas flow has a temperature greater than or equal to         a temperature threshold value, wherein the exhaust gas bypass         and/or flow control valve is arranged directly upstream of the         TT-SCR.

The novel SCR system and the method for treating an exhaust gas flow comprising the novel SCR system are explained below. The invention encompasses all the embodiments listed both individually and in combination with one another.

An “exhaust gas flow” within the meaning of the present invention relates to an exhaust gas flow from an internal combustion engine, irrespective of the burned fuel. The internal combustion engines can be gasoline engines or lean burn engines, for example.

Advantageously, lean burn engines are diesel engines. The term “diesel engines” encompasses both light-duty diesel engines (LDD) as well as heavy-duty diesel engines (HDD).

Surprisingly, it has been found that certain HT-SCR catalysts which are described below and which contain Cu- or Fe-based zeolites as catalytically active component, not only efficiently reduce nitrogen oxides in normal to high temperature ranges to nitrogen, but also quantitatively store sulfur oxides SO_(x) in low to medium temperature ranges, and thus can effectively protect a downstream TT-SCR from contamination by SO_(x). The present invention makes use of this effect in that the exhaust gas flow is basically first conducted through the HT-SCR. It also stores SO_(x) in low temperature ranges and thus removes this component from the exhaust gas flow. This prevents contamination of the downstream TT-SCR by SOx. In low temperature ranges, the HT-SCR thus serves as an SO_(x) trap, while the nitrogen oxides NO_(x) are reduced completely or at least largely in the TT-SCR, since the HT-SCR has no or only very low NO_(x) conversion rates in low temperature ranges. In contrast, in the normal to high temperature range, the HT-SCR not only continues to act as an SO_(x) trap, but additionally also assumes the NO_(x) conversion. When a temperature threshold value is exceeded, the exhaust gas flow is conducted past the TT-SCR via a bypass. This operating strategy offers several advantages. On the one hand, the TT-SCR is prevented from being damaged by the hot exhaust gas flow. On the other hand, the TT-SCR is protected by such measure from SO_(x) emissions, which are released from the HT-SCR in high temperature ranges or can no longer be stored completely due to the elevated temperatures. The selection of the temperature threshold value for a particular embodiment of the present invention results from the actual exhaust gas temperature; the exhaust gas mass flow, the sulfur concentration in the exhaust gas, the size of the HT-SCR, and the temperature sensitivity of the TT-SCR. These factors are essentially determined by the final application. Therefore, the temperature threshold value is also a variable which depends on the respective application and is determined by the end user. The person skilled in the art is aware of the described phenomenon and can determine the temperature threshold value, without departing from the scope of protection of the present invention. The temperature threshold value is advantageously at a temperature above 250° C. or greater than or equal to the light-off temperature of the HT-SCR.

In this case, a “low” temperature range means temperatures of 60° C. to less than 250° C., a “medium” temperature range means temperatures of 250° C. to less than 450° C. and a “high” temperature range means temperatures of 450° C. to 700° C.

In the context of the present invention, the term “contamination” of a catalyst means that a substance acts on a catalyst in such a way that it reduces or cancels the effect of the catalyst. In this context, “permanent” means “lasting a long period of time, consistent, long lasting.” A “permanent contamination of a catalyst” is accordingly an action of a substance on a catalyst in such a way that the effect of this catalyst is reduced or canceled for a long period of time, consistently, long lastingly and this effect is not reversible.

The SCR system according to the invention effectively reduces nitrogen oxides from exhaust gases from internal combustion engines in low, medium and high temperature ranges. In this context, “effective” means that the nitrogen oxides are effectively and efficiently reduced.

Furthermore, the SCR system according to the invention has high activity, selectivity and temperature stability. As stated above, the SCR system according to the invention comprises an HT-SCR and a TT-SCR, i.e., two catalysts. The person skilled in the art is aware that the performance of a catalyst is evaluated by the extent to which it increases the speed of a chemical reaction (catalyst activity), influences its course (catalyst selectivity) and by how long its effectiveness lasts (catalyst life). Catalyst activity is thus a measure for how quickly the catalyst converts reactants to products. In the case of SCR catalysts, the reactants are nitrogen oxides (NO_(x)) and reductants, and the products are nitrogen (N₂) and water. On the other hand, catalyst selectivity describes the phenomenon that one of a plurality of possible reaction products is preferably formed during a reaction. In the case of SCR catalysts, both N₂ and N₂O can be formed during the reaction of NO_(x) with reductants, wherein N₂ is desirable and N₂O is undesirable. SCR catalysts with high selectivity for the formation of N₂ are therefore advantageous.

A “temperature-stable” catalyst is heat-resistant even at relatively high temperatures. The resistance here relates primarily to the structure of the catalytically active coating and of the carrier substrate, both of which are explained in more detail below. However, in the context of catalysts, “temperature stability” is also defined specifically to the application as the ability to carry out a certain function. In the context of the present invention, “temperature-stable” SCR systems are arrangements comprising an HT-SCR and a TT-SCR, wherein the SCR systems are designed such that they are heat-resistant. on the one hand, and reduce nitrogen oxides to nitrogen for the mentioned low, medium and high temperature ranges, on the other hand. This is achieved by conducting the exhaust gas, depending on the temperature threshold value and the current exhaust gas temperature, either only through the upstream HT-SCR or first through the HT-SCR and then through the downstream TT-SCR.

In the context of the present invention, both the HT-SCR and the TT-SCR are advantageously present in the form of a catalytically active coating on a carrier substrate. Carrier substrates can be so-called flow-through substrates or wall-flow filters. Both can consist of inert materials, for example ceramic materials, such as silicon carbide, aluminum titanate or cordierite. Alternatively, in the case of a flow-through substrate, the inert material may be metal substrates. In the context of the present invention, the carrier substrates of both SCRs can consist of inert materials, or both consist of metal substrates, or one of the two SCRs has a carrier substrate made of an inert ceramic material and the other has a carrier substrate made of metal. The mentioned carrier substrates are known to the person skilled in the art and are commercially available.

However, the carrier substrates themselves can also be catalytically active and contain catalytically active material, for example SCR catalytically active material.

Cu- and/or Fe-containing zeolites are used as SCR catalytically active materials for the HT-SCR catalysts to be used according to the invention.

As SCR catalytically active materials for the TT-SCR, mixed oxide-based materials known to the person skilled in the art and containing manganese compounds are used.

These carrier substrates contain a matrix component in addition to the catalytically active material. All inert materials that are also otherwise used to produce catalyst substrates can be used as matrix components. These are, for example, silicates, oxides, nitrides or carbides, wherein in particular magnesium aluminum silicates are preferred.

In other embodiments of the catalyst according to the invention, it is itself present as part of a carrier substrate, i.e., for example, of a flow-through substrate or wall-flow filter. Such carrier substrates additionally contain the matrix components already described above.

Carrier substrates containing catalysts according to the invention can be used as such in exhaust gas purification. However, they can also in turn be coated with catalytically active materials, for example SCR catalytically active materials. If such materials are to be SCR catalytically active, the aforementioned SCR catalysts come into consideration.

In order to produce catalytically active carrier substrates, a mixture consisting of, for example, 10 to 95% by weight of an inert matrix component and 5 to 90% by weight of catalytically active material is, for example, extruded according to methods known per se. As already described above, all inert materials that are also otherwise used to produce catalyst substrates can be used as matrix components. These are, for example, silicates, oxides, nitrides or carbides, wherein in particular magnesium aluminum silicates are preferred.

Applying the catalyst according to the invention to the inert or itself catalytically active carrier substrate and applying a catalytically active coating to a carrier substrate comprising a catalyst according to the invention may be carried out by methods known to the person skilled in the art, thus, for instance, according to the usual dip coating methods or pump and vacuum coating methods with subsequent thermal aftertreatment (calcination).

The person skilled in the art knows that, in the case of wall-flow filters, the latter's average pore size and the average particle size of the catalyst according to the invention can be adapted to each other such that the resulting coating lies on the porous walls that form the channels of the wall-flow filter (on-wall coating). However, average pore size and average particle size are preferably adapted to one another such that the catalyst according to the invention is located in the porous walls that form the channels of the wall-flow filter, that a coating of the inner pore surfaces thus takes place (in-wall coating). In this case, the average particle size of the catalyst according to the invention must be small enough to penetrate into the pores of the wall-flow filter.

Hereinafter, the terms “catalytically active coating” and “catalytically active layer” are used synonymously.

Advantageously, the HT-SCR to be used according to the invention is present in the form of a catalytically active layer on a carrier substrate, wherein the catalytically active layer is a molecular sieve selected from

-   -   an aluminosilicate having a SAR of 5-50 and a silicon aluminum         phosphate (SAPO) having an (Al+P)/Si value of 4-15 is selected,     -   wherein the molecular sieve contains 1-10% by weight of a         transition metal selected from Fe, Cu and mixtures thereof,         calculated as Fe₂O₃ and CuO respectively, based on the total         weight of the molecular sieve,     -   and wherein the molecular sieve contains alkali metal and         alkaline earth metal cations selected from Li, Na, K, Rb, Cs,         Mg, Ca, Sr, Ba, and mixtures thereof in a total amount of ≤1% by         weight, calculated in the form of the pure metals and based on         the total weight of the molecular sieve,     -   and wherein the molecular sieve contains the metals Co, Mn, Cr,         Zr and Ni in a total amount of ≤1% by weight, calculated in the         form of the pure metals and based on the total weight of the         molecular sieve.

The term “SAR” in this case stands for “silica-to-alumina ratio”, i.e., for the molar ratio of SiO₂ to Al₂O₃ of an aluminosilicate zeolite.

On the other hand, silicon aluminum phosphates, also referred to as “SAPOs,” indicate the (Al+P)/Si value. This is the sum of the amounts of aluminum and phosphorus divided by the amount of silicon.

If the molecular sieve is a SAPO, its content of Al₂O₃ is 30 to 45% by weight, preferably 36 to 42% by weight, and particularly preferably 38% by weight. The content of P₂O₅ is 30 to 50% by weight, preferably 42 to 50% by weight, and particularly preferably 49% by weight. The SiO₂ content is 5 to 20% by weight, preferably 9 to 15% by weight, and particularly preferably 10% by weight. From such ranges for the contents of Al₂O₃, P₂O₅, and SiO₂, (Al+P)/Si values of 4 to 15, preferably 6 to 10, particularly preferably 8 to 9, and most preferably of 8.6 arise.

If the molecular sieve is an aluminosilicate, the SAR is 5 to 50, preferably 10 to 35, particularly preferably 12 to 30, and most preferably 30.

The molecular sieve contains 1 to 10% by weight, preferably 1 to 9% by weight, particularly preferably 2.5 to 7% by weight, and most preferably 3.5 to 4.5% by weight of a transition metal selected from iron, copper, and mixtures thereof, calculated as Fe₂O₃ and CuO respectively, and based on the total weight of the molecular sieve.

In one embodiment, the molecular sieve contains 1 to 10% by weight Fe, preferably 3 to 9% by weight Fe, particularly preferably 4 to 7% by weight Fe, and most preferably 3.5% by weight Fe, in each case calculated as Fe₂O₃ and based on the total weight of the molecular sieve.

In a further embodiment, the molecular sieve contains 1 to 10% by weight Cu, preferably 1 to 7% by weight Cu, particularly preferably 2.5 to 4% by weight Cu, and most preferably 4.5% by weight Cu, in each case calculated as CuO and based on the total weight of the molecular sieve.

Furthermore, the molecular sieve contains alkali metal and alkaline earth metal cations selected from Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, and mixtures thereof in a total amount of ≤1% by weight, calculated in the form of the pure metals and based on the total weight of the molecular sieve. The total amount of these alkali metal and alkaline earth metal cations is preferably 0 to 0.7% by weight, particularly preferably 0 to 0.5% by weight, and most preferably 0.005 to 0.4% by weight.

In one embodiment of the present invention, the molecular sieve constituting the catalytically active layer of the HT-SCR is SAPO-34. The (Al+P)/Si value and the contents of CuO and/or Fe₂O₃, the alkali metal and alkaline earth metal content, and the content of the metals Co, Mn, Cr, Zr and Ni correspond to the above specifications.

In a further embodiment of the present invention, the molecular sieve constituting the catalytically active layer of the HT-SCR is selected from small-pore zeolites having a maximum pore size of eight tetrahedral atoms and beta zeolite. The contents of CuO and/or Fe₂O₃, the alkali metal and alkaline earth metal content and the content of the metals Co, Mn, Cr, Zr and Ni correspond to the above specifications.

The person skilled in the art is aware that zeolites are classified by their pore size. The pore size is defined by the ring size of the largest pore opening. Large-pore zeolites have a maximum ring size of 12 tetrahedral atoms, medium-pore zeolites have a maximum ring size of 10 and small-pore zeolites have a maximum ring size of 8 tetrahedral atoms. Small-pore zeolites are, for example, ACO, AEI, AEN, AFN, AFT, AFX, ANA, APC, APD, ATT, CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI, GIS, GOO, IHW, ITE, ITW, LEV, KFI, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SIV, THO, TSC, UEI, UFI, VNI, YUG and ZON.

Zeolites having a medium pore size are, for example, FER, MFI, ZSM-57 and SUZ-4.

Large-pore zeolites are, for example, FAU, BEA, ZSM-3, ZSM-4, ZSM-10, ZSM-12, ZSM-20, zeolite omega, zeolite L, zeolite X, REY, USY, RE-USY and MOR.

In a particularly advantageous embodiment, the HT-SCR is an Fe-BEA having a SAR of 5 to 30, an alkali metal content of 0 to 0.7% by weight, particularly preferably 0 to 0.5% by weight, calculated as pure metals, and an Fe content of 3 to 9% by weight, particularly preferably 4.5% by weight, calculated as Fe₂O₃, wherein the alkali metal and Fe contents are each based on the total weight of the zeolite.

In a further particularly advantageous embodiment, the HT-SCR is a Cu-CHA having a SAR of 10 to 35, an alkali metal content of 0 to 07% by weight, particularly preferably 0 to 0.5% by weight, calculated as pure metals, and a Cu content of 1 to 7% by weight, particularly preferably 3.5% by weight, calculated as CuO, wherein the alkali metal and Cu contents are each based on the total weight of the zeolite.

In a further embodiment, the molecular sieve is applied as one or more layers to a flow-through honeycomb body. Binders, e.g., selected from SiO₂, Al₂O₃, ZrO₂, or combinations thereof, may additionally be contained in the layers. Suitable binders also include boehmite and silica gel.

The TT-SCR to be used according to the invention advantageously comprises a monolithic flow-through substrate with a manganese-containing coating as catalytically active layer.

Advantageously, the manganese-containing coating is a manganese-containing mixed oxide selected from

-   -   mixed oxides of the general formula Mn_(a)Me_(1-a)O_(b), where         Me is one or more elements from the group Fe, Co, Ni, Cu, Zr,         Nb, Mo, W, Ag, Sn, Ce, Pr, La, Nd, Ti and Y and where         a=0.02-0.98 and b=1.0-2.5,     -   mixed oxides of the general formula Mn_(w)Ce_(x)Me_(1-w-x)O_(y),         where Me is one or more elements from the group Fe, Co, Ni, Cu,         Zr, Nb, Mo, W, Ag, Sn, Pr, La, Nd, Ti and Y and where         w=0.02-0.98, x=0.02-0.98 and y=1.0-2.5,     -   spinels of the general formula MnMe₂O₄ or MeMn₂O₄, where Me is         Fe, Al, Cr, Co, Cu or Ti.

If the manganese-containing coating is a mixed oxide of the general formula Mn_(a)Me_(1-a)O_(b), Me is selected from the group Fe, Co, Ni, Cu, Zr, Nb, Mo, W, Ag, Sn, Ce, Pr, La, Nd, Ti and Y. Here, a=0.02-0.98 and b=1.0-2.5. Me is particularly advantageously selected from Fe, Cu, Nb, Mo, W, Sn and Ti.

If the manganese-containing coating is a mixed oxide of the general formula Mn_(w)Ce_(x)Me_(1-w-x)O_(y), Me is selected from the group Fe, Co, Ni, Cu, Zr, Nb, Mo, W, Ag, Sn, Ce, Pr, La, Nd, Ti and Y. Here, w=0.02-0.98, x=0.02-0.98 and y=1.0-2.5. Me is particularly advantageously selected from Fe, Cu, Nb, Mo, W, Sn and Ti.

The components of the exhaust gas aftertreatment system which are or may be present in addition to the HT-SCR and TT-SCR are explained below. If a DPF is mentioned, it can be a DPF, SDPF or CDPF. FIGS. 1a and 1b show by way of example the basic principle of the present invention, namely the arrangement of HT-SCR and TT-SCR in the bypass. FIGS. 2a to 5 show further embodiments of the invention, in which further components of the exhaust gas purification system are shown.

According to the invention, a reductant supply system is arranged directly upstream of the HT-SCR. In one embodiment, the reductant supply system comprises a reductant source, a reductant pump, and a reductant dispenser or a reductant injector (not shown). The exact design of the reductant supply system may also be determined by the type of reductant. For example, in a further embodiment, a pump can be dispensed with when liquids are metered if the container or tank containing the reductant is attached in such a way that the delivery to the reductant dispenser or the reductant injector is brought about by gravity. The person skilled in the art knows the various technical solutions and can use any reductant supply systems, without departing from the scope of protection of the claims.

The reductant source may be a container or tank which can receive a reductant, such as ammonia (NH₃), urea or another NH₃ storage source, such as carbamates; formates or also metal salts (e.g., Sn(NH₃)₈Cl₂), which decompose, forming ammonia.

The reductant source is advantageously selected from aqueous solutions of ammonia, urea, ammonium carbamate, ammonium formate, ammonium acetate, ammonium propionate, guanidinium formate, methanamide and mixtures of the specified aqueous solutions.

Alternatively, solid nitrogen-containing reductant sources which are evaporated to reductive or reactive constituents, in particular ammonia (NH₃), can also be used. Suitable solids for this purpose are, for example, ammonium carbamate, ammonium carbonate, ammonium formate, ammonium acetate, ammonium propionate and metal amine salts. The use of such solids and a process for the generation of a gaseous reductant for the reduction of nitrogen oxides in oxygen-containing exhaust gases from a solid reductant are described, for example, in DE 101 01 364 A1 and WO 2008/119492 A1.

The reductant source is connected to the pump in such a way that is delivers reductant, wherein the pump is designed to pump reductant from the reductant source to the dispenser. As shown in FIG. 1 a, for example, the dispenser can comprise a reductant injector or a reductant metering device arranged upstream of the bypass valve 150 and the HT-SCR catalyst 120. In alternative embodiments, the reductant injector may be located at other locations of the aftertreatment system, such as upstream of the DOC in FIGS. 4a and 4b . In the illustrated embodiment, the injector may be selectively controlled to inject reductant directly into the exhaust gas flow prior to the exhaust gas flow passing through the bypass valve 150.

In a further embodiment, such as in FIG. 2, the reductant supply system may comprise more than one reductant injector, whereby HT-SCR 220 and TT-SCR 230 may be supplied with reductant independently of each other. This operating mode is particularly advantageous in order to allow the TT-SCR a very short light-off time, since the reductant does not have to flow through the HT-SCR beforehand and is possibly adsorbed there.

In all embodiments, the reductant in the SCR catalysts reacts with NO_(x) to reduce it to harmless emissions N₂ and H₂O.

The engine system 110 furthermore comprises various physical and/or virtual sensors, such as exhaust gas temperature sensors. Advantageously, such sensors are arranged downstream of the engine 110 and upstream of the turbocharger as well as upstream of the DOC and DPF (see FIGS. 2, 3 a, 3 b, 4 a and 4 b). For the regulation of the bypass valve 150 and a possible reductant metering downstream of the HT-SCR or upstream of the TT-SCR, it is advantageous to use a temperature sensor 140 upstream of the bypass and/or flow control valve. The exhaust gas temperature sensor 140 is arranged downstream of the HT-SCR 120 and upstream of the bypass valve 150. The exhaust gas temperature sensors may be physical sensors and/or virtual sensors designed to determine (e.g., detect, estimate, or simulate) a temperature of exhaust gas exiting the engine 110 or a temperature of exhaust gas exiting the HT-SCR 120. Although not shown, the engine system 110 may contain various other sensors, such as a differential pressure sensor for determining a pressure difference on either side of the DPF in FIGS. 2, 3 a, 3 b, 4 a and 4 b, mass flow sensors for determining the mass-related exhaust gas flow rate, and exhaust gas property sensors for determining the mass-related concentrations of various compounds in the exhaust gas, such as NO_(x), oxygen, nitrogen, ammonia, and the like. The various sensors may be expediently arranged throughout the entire engine system 110 and may be in communication with the controller to monitor, control, and regulate the operating conditions of the system.

Exhaust Gas Bypass Valve or Flow Control Valve:

According to some implementations of the system, the exhaust gas bypass and/or flow control valve is designed to conduct the entire exhaust gas flow through the low-temperature SCR catalyst, if a temperature of the exhaust gas flow is below the minimum operating temperature of the HT-SCR catalyst and the HT-SCR still has no high conversion rates. The minimum operating temperature may be lower than the temperature threshold value. The exhaust gas bypass and/or flow control valve may gradually decrease the amount of the exhaust gas flow that flows through the low-temperature SCR catalyst from 100% to 0% of the exhaust gas flow, if the temperature of the exhaust gas flow rises accordingly from the minimum operating temperature to the temperature threshold value. Similarly, the bypass valve may gradually increase the amount of exhaust gas flow that flows through the low-temperature SCR catalyst from 0% to 100% of the exhaust gas flow, if the temperature of the exhaust gas flow decreases accordingly from the temperature threshold value to the minimum operating temperature.

Optionally, the exhaust gas bypass and/or flow control valve may be electronically connected to a controller that selectively controls the switching of the valve. The controller is not shown in FIGS. 1 to 5. The exhaust gas bypass and/or flow control valve may be switched between an open position and a closed position. In the open position, i.e., below the minimum operating temperature of the HT-SCR, the exhaust gas bypass and/or flow control valve conducts all of the exhaust gas into the TT-SCR. In contrast, in the closed position, i.e., above the minimum operating temperature of the HT-SCR, the exhaust gas bypass and/or flow control valve does not conduct any partial amounts of the exhaust gas into the TT-SCR. In other words, the exhaust gas bypass and/or flow control valve in the closed position results in the TT-SCR being bypassed. In certain implementations, the exhaust gas bypass and/or flow control valve may be adjusted to any positions between the open and the closed position to selectively regulate the flow rate to the TT-SCR. In other words, the exhaust gas bypass and/or flow control valve may be controlled such that any partial amounts of exhaust gas from the HT-SCR flow into the TT-SCR catalyst. The gas path, i.e., the flow direction of the exhaust gas, is illustrated schematically in FIGS. 6a and 6 b.

The minimum operating temperature here is the temperature that is at least required in order to initiate the provision of the reductant and the actual catalytic reaction.

Advantageously, in all embodiments described in the context of the present invention, a further reductant supply system is arranged directly upstream of the TT-SCR.

In an advantageous embodiment, the reductant supply system contains a) a reductant source, b) a reductant pump, and c) a reductant dispenser or a reductant injector.

In a further advantageous embodiment, a pump is dispensed with when metering the liquid reductant. Instead, the container or tank containing the reductant is attached such that the delivery to the reductant dispenser or the reductant injector is brought about by gravity.

In all embodiments of the present invention, a reductant supply system is located directly upstream of the HT-SCR, see FIGS. 1a and 1 b, for example. In FIGS. 3b, 4a and 5b , the SDPF has the function of an HT-SCR.

Optionally, further reductant supply systems may also be present. Such further reductant supply systems are advantageously arranged directly upstream of the TT-SCR, as shown in FIGS. 2a, 2b, 3a, 3b and 5.

In advantageous embodiments, an oxidation catalyst is located directly upstream of the reductant supply system located directly upstream of the HT-SCR.

Oxidation catalysts have been known for a long time in the prior art and described in a wide range of embodiments. In most cases, the precious metals platinum and/or palladium are used as oxidation-catalytically active components (see, for example, US 2011/0206584 A1); however, catalysts additionally containing gold have also already been described, for example in the exhaust gas aftertreatment system according to EP 1 938 893 A2. Oxidation catalysts are known to the person skilled in the art and can be used within the scope of the present invention, without departing from the scope of protection of the patent application.

The inert carrier substrates known to the person skilled in the art are in particular used according to the present invention as carrier substrates. In particular, these are honeycomb bodies made of metal or preferably of ceramic, which can be designed as flow-through honeycomb bodies or else as wall-flow filter bodies. Ceramic honeycomb bodies made of cordierite are preferred.

In a further embodiment, a further HT-SCR is located between the HT-SCR, upstream of which a reductant supply system is located, and the temperature sensor. In this case, the different HT-SCR units can represent a combination of various technologies. For example, combinations of Fe zeolite, for example Fe-BEA, and Cu zeolite, for example Cu-SSZ-13, as well as combinations of vanadium-based systems and Cu zeolite, on the one hand, and also the combination of various Cu zeolite technologies, on the other hand, can be used. Here as well, the already mentioned flow-through substrates are used. Furthermore, the HT-SCR can also be an SDPF, i.e., an SCR which is integrated on a DPF. All combinations mentioned here are known to the person skilled in the art and can be used, without departing from the scope of protection of the claims.

In a further embodiment, a catalytically coated diesel particulate filter (CDPF) is located between the oxidation catalyst and the reductant supply system located directly upstream of the HT-SCR.

In order to remove particulate emissions from the exhaust gas of diesel vehicles, special particulate filters are used, which can be provided with an oxidation-catalytically active coating to improve their properties. Such a coating serves to reduce the activation energy for the oxygen-based particle combustion (soot combustion) and hence to reduce the soot ignition temperature on the filter, to improve the passive regeneration behavior by oxidation of nitrogen monoxide contained in the exhaust gas into nitrogen dioxide, and to suppress breakthroughs of hydrocarbon and carbon monoxide emissions. Suitable carrier substrates for such CDPFs are, for example, wall-flow filters of silicon carbide, aluminum titanate and cordierite. Such particulate filters are comprehensively described, for example, in M Pfeifer, M Votsmeier, M Kögel, P C Spurk and E S Lox: “The Second Generation of Catalyzed Diesel Particulate Filters for Passenger Cars—Particulate Filters with Integrated Oxidation Catalyst Function,” SAE Paper SAE 2005-01-1756.

In a further embodiment, one or more HT-SCRs are arranged upstream of the oxidation catalyst. In this case, a reductant supply system is located directly upstream of the HT-SCR closest to the internal combustion engine.

In a further embodiment, one or more HT-SCRs and an ASC are arranged upstream of the oxidation catalyst. In this case, a reductant supply system is located directly upstream of the HT-SCR closest to the internal combustion engine. In this arrangement of ASC and one or more HT-SCRs, which are all arranged upstream of the oxidation catalyst, the ASC is located in the most downstream position. Advantageously, the ASC is then located directly upstream of the oxidation catalyst.

In a further embodiment, the HT-SCR with downstream TT-SCR is directly on the engine output side. In this case, a reductant supply system is located directly upstream of the HT-SCR. The exhaust gas bypass and/or flow control valve described above is located downstream of the HT-SCR. In an advantageous embodiment, a further temperature sensor is located upstream of the exhaust gas bypass and/or flow control valve. A further reductant supply system can also be arranged directly upstream of the TT-SCR in this configuration close to the engine. The shown structure corresponds to FIGS. 1a and 1b . In a particularly advantageous embodiment, an ammonia slip catalyst is arranged downstream of the TT-SCR.

In order to improve the conversion of nitrogen oxides in the second SCR catalyst, it may be necessary to meter the ammonia in an amount which is approximately 10 to 20% above the amount required per se, i.e., the stoichiometric amount. However, this increases the risk of a higher secondary emission, in particular due to increased ammonia slip. Since ammonia has a pungent odor even in low concentration and is legally limited in the commercial vehicle sector, the ammonia slip must be minimized. For this purpose, so-called ammonia barrier catalysts are known, which are arranged downstream of an SCR catalyst in the flow direction of the exhaust gas in order to oxidize ammonia breaking through. Ammonia barrier catalysts in various embodiments are described, for example, in U.S. Pat. No. 5,120,695, WO 02/100520 A1 and EP 0 559 021 A2.

In a further embodiment, the catalyst arrangement according to the invention therefore comprises an ammonia barrier catalyst that follows the second SCR catalyst.

For example, the ammonia barrier catalyst comprises SCR active material and one or more platinum group metals, in particular platinum or platinum and palladium. In particular, all SCR catalysts described above are suitable as SCR catalytically active material.

The object of the invention of providing a method for treating an exhaust gas flow comprising the novel SCR system is achieved according to the invention by

-   -   providing an exhaust gas aftertreatment system comprising an SCR         catalyst for medium to high temperatures (HT-SCR), a         low-temperature SCR (TT-SCR) arranged downstream of the SCR for         low to medium temperatures, a temperature sensor arranged         directly downstream of the HT-SCR and an exhaust gas bypass         and/or flow control valve arranged directly upstream of the         low-temperature SCR,         -   wherein the SCR catalyst for medium to high temperatures,             the medium temperature range comprising temperatures of             250° C. to less than 450° C. and the high temperature range             comprising temperatures of 450° C. to 750° C., is designed             both             -   a) to reduce NO_(x) in an exhaust gas having a                 temperature above a temperature threshold value,                 -   and             -   b) to store the SO_(x) contained in the exhaust gas                 across the temperature range of the exhaust gas, which                 is below a temperature threshold value,         -   and the low-temperature SCR is designed to reduce NO_(x) in             an exhaust gas having a temperature below a temperature             threshold value, wherein the low temperature range comprises             temperatures of 60° C. to less than 250° C.,         -   and wherein the temperature sensor measures the temperature             of the exhaust gas flow exiting the HT-SCR,         -   and wherein the exhaust gas bypass and/or flow control valve             is designed to conduct the exhaust gas flow in its entirety             past the low-temperature SCR, if such exhaust gas flow has a             temperature greater than or equal to a temperature threshold             value,     -   storing the SO_(x) contained in the exhaust gas in the HT-SCR,     -   reducing NO_(x) in an exhaust gas flow with the low-temperature         SCR catalyst, if a temperature of the exhaust gas flow is within         a low temperature range;     -   conducting the exhaust gas stream in its entirety past the         low-temperature SCR, if such exhaust gas flow has a temperature         greater than or equal to a temperature threshold value, and     -   reducing NO_(x) in an exhaust gas flow with the SCR catalyst for         normal to high temperatures, if the temperature of the exhaust         gas flow is within a normal to high temperature range.

DESCRIPTION OF THE DRAWINGS

In the drawings, arrows always stand for the exhaust gas flow from upstream to downstream, i.e., from near the engine to near the exhaust pipe.

Dashed arrows, for example between the two components engine 110 and SCR 120 in FIG. 1a and FIG. 1b mean that further components can optionally be located between such components.

Dotted arrows represent the conducting of the entire exhaust gas flow past the low-temperature SCR, if such exhaust gas flow has a temperature above a temperature threshold value, see 180, 280, 380, 480 and 580 in FIGS. 1 to 5.

The abbreviations used in the drawings have the following meanings:

-   -   HT-SCR Catalyst for selective catalytic reduction (SCR); here:         SCR catalyst for medium to high temperatures     -   TT-SCR Low-temperature SCR     -   OX Oxidation catalyst     -   DPF Diesel particulate filter     -   SDPF Diesel particulate filter (DPF) coated with an SCR     -   CDPF Catalytically coated diesel particulate filter=DPF coated         with an oxidation catalyst     -   ASC Ammonia slip catalyst

The catalytic systems, i.e., the HT-SCR, the TT-SCR, the OX, the DPF, the SDPF and the ASC, can each be composed independently of each other of one or any number of bricks of different dimensions and shape.

FIG. 1a shows the core of the present invention. An HT-SCR catalyst 120 for medium to high temperatures is located downstream of the engine 110. A reductant supply system 160 which comprises a reductant source, a reductant pump, and a reductant dispenser or a reductant injector (not shown), is located directly upstream of such SCR catalyst 120. An exhaust gas temperature sensor 140 is located downstream of the SCR 120. An exhaust gas bypass and/or flow control valve 150, which communicates with the SCR 120 in such a way that it receives exhaust gas, is located downstream of the exhaust gas temperature sensor 140. The exhaust gas bypass and/or flow control valve 150 conducts the exhaust gas flow past the low-temperature SCR 130 via the bypass 180, if the exhaust gas flow has a temperature greater than or equal to a temperature threshold value. However, if the temperature of the exhaust gas flow is below such temperature threshold value, the exhaust gas flow is conducted, completely or partially, through the low-temperature SCR 130, see arrow 170.

In the embodiment according to FIG. 1 b, a further reductant supply system 190 is also located between exhaust gas bypass and/or the flow control valve 150 and the low-temperature SCR 130. In the event that the temperature of the exhaust gas flow is below the temperature threshold value, the exhaust gas flow is conducted through the low-temperature SCR and, immediately before this introduction, the low-temperature SCR is supplied with reductant.

FIG. 2 shows a preferred embodiment of an exhaust gas purification system comprising the combination according to the invention of an SCR for normal to high temperatures and a low-temperature SCR. This system comprises an oxidation catalyst 215. A reductant supply system 260, which is constructed like the corresponding system 160 in FIG. 1a and FIG. 1b and supplies reductant to the SDPF 225 is located directly downstream of such oxidation catalyst 215. An SCR 220 is located directly downstream of the SDPF 225. An exhaust gas temperature sensor 240 is arranged downstream of the SCR 220. An exhaust gas bypass and/or flow control valve 250, which communicates with the SCR 220 in such a way that it receives exhaust gas, is located downstream of exhaust gas temperature sensor 240. The exhaust gas bypass and/or flow control valve 250 conducts the exhaust gas flow past the low-temperature SCR 230 via the bypass 280, if the exhaust gas flow has a temperature greater than or equal to a temperature threshold value. However, if the temperature of the exhaust gas flow is below such temperature threshold value, the exhaust gas flow is conducted, partially or completely, through the low-temperature SCR 230, see arrow 270. Optionally, a further reductant supply system 290 may also be provided between the exhaust the gas bypass and/or the flow control valve 250 and the low-temperature SCR 230 as described in FIG. 1 b. In the event that the temperature of the exhaust gas flow is below the temperature threshold value, the exhaust gas flow is conducted through the low-temperature SCR and, immediately before this introduction, the low-temperature SCR is supplied with reductant.

FIG. 3a shows another advantageous embodiment of an exhaust gas purification system containing the combination according to the invention of an SCR for normal to high temperatures and a low-temperature SCR. This system comprises an oxidation catalyst 315 and a CDPF 316 located directly downstream thereof. A reductant supply system 360, which is constructed like the corresponding system 160 in FIG. 1a and FIG. 1b and supplies reductant to the SCR 225, is located downstream of the CDPF. An exhaust gas temperature sensor 340 is arranged downstream of the SCR 320. An exhaust gas bypass and/or flow control valve 350, which communicates with the SCR 320 in such a way that it receives exhaust gas, is located downstream of the exhaust gas temperature sensor 340. The exhaust gas bypass and/or flow control valve 350 conducts the exhaust gas flow past the low-temperature SCR 330 via the bypass 380, if the exhaust gas flow has a temperature greater than or equal to a temperature threshold value. However, if the temperature of the exhaust gas flow is below such temperature threshold value, the exhaust gas flow is conducted through the low-temperature SCR 330, see arrow 370. Optionally, a further reductant supply system 390 may also be provided between the exhaust gas bypass and/or the flow control valve 350 and the low-temperature SCR 330 as described in FIG. 1b . In the event that the temperature of the exhaust gas flow is below the temperature threshold value, the exhaust gas flow is conducted through the low-temperature SCR and, immediately before this introduction, the low-temperature SCR is supplied with reductant.

FIG. 3b shows another advantageous embodiment of the present invention. The system comprises an oxidation catalyst 315, followed by a reductant supply system 360, which is constructed like the corresponding system 160 in FIG. 1a and FIG. 1b and supplies reductant to the SDPF 325. An exhaust gas temperature sensor 340 is arranged downstream of the SDPF 325. An exhaust gas bypass and/or flow control valve 350, which communicates with the SDPF 325 in such a way that it receives exhaust gas, is located downstream of the exhaust gas temperature 340. The exhaust gas bypass and/or flow control valve 350 conducts the exhaust gas flow past the low-temperature SCR 330 via the bypass 380, if the exhaust gas flow has a temperature greater than or equal to a temperature threshold value. However, if the temperature of the exhaust gas flow is below such temperature threshold value, the exhaust gas flow is conducted through the low-temperature SCR 330, see arrow 370. Optionally, a further reductant supply system 390 may also be provided between the exhaust gas bypass and/or the flow control valve 350 and the low-temperature SCR 330 as described in FIG. 1b . In the event that the temperature of the exhaust gas flow is below the temperature threshold value, the exhaust gas flow is conducted through the low-temperature SCR and, immediately before this introduction, the low-temperature SCR is supplied with reductant.

FIG. 4a relates to a further embodiment based on the system described in FIG. 3a . A reductant supply system 465, which is constructed like the corresponding system 160 in FIG. 1a and FIG. 1b and supplies reductant to the SCR 425, is located upstream of the oxidation catalyst 415 and a CDPF 416 located directly downstream thereof. As described in FIG. 3a , a further reductant supply system 460, which is constructed like the corresponding system 160 in FIG. 1a and FIG. 1b and supplies reductant to the SCR 420, is arranged downstream of the oxidation catalyst 415 and the CDPF 416. An exhaust gas temperature sensor 440 is arranged downstream of the SCR 420. An exhaust gas bypass and/or flow control valve 450, which communicates with the SCR 420 in such a way that it receives exhaust gas, is located downstream of the exhaust gas temperature sensor 440. The exhaust gas bypass and/or flow control valve 450 conducts the exhaust gas flow past the low-temperature SCR 430 via the bypass 480, if the exhaust gas flow has a temperature greater than or equal to a temperature threshold value. However, if the temperature of the exhaust gas flow is below such temperature threshold value, the exhaust gas flow is conducted through the low-temperature SCR 430, see arrow 470. Optionally, a further reductant supply system 490 may also be provided between the exhaust gas bypass and/or the flow control valve 450 and the low-temperature SCR 430 as described in FIG. 1 b. In the event that the temperature of the exhaust gas flow is below the temperature threshold value, the exhaust gas flow is conducted through the low-temperature SCR and, immediately before this introduction, the low-temperature SCR is supplied with reductant.

FIG. 4b relates to a further embodiment based on the system described in FIG. 4a . Here as well, as in FIG. 4a , a reductant supply system 465, which is constructed like the corresponding system 160 in FIG. 1a and FIG. 1b and supplies reductant to the two SCR 425 located one after the other, is located upstream of the oxidation catalyst 415. A reductant supply system 465, which is constructed like the corresponding system 160 in FIG. 1a and FIG. 1b and supplies reductant to the SDPF 420, is downstream of the oxidation catalyst 415. An exhaust gas temperature sensor 440 is arranged downstream of the SDPF 420. An exhaust gas bypass and/or flow control valve 450, which communicates with the SDPF 420 in such a way that it receives exhaust gas, is located downstream of the exhaust gas temperature sensor 440. The exhaust gas bypass and/or flow control valve 450 conducts the exhaust gas flow past the low-temperature SCR 430 via the bypass 480. if the exhaust gas flow has a temperature greater than or equal to a temperature threshold value. However, if the temperature of the exhaust gas flow is below such temperature threshold value, the exhaust gas flow is conducted through the low-temperature SCR 430, see arrow 470. Optionally, a further reductant supply system 390 may also be provided between the exhaust gas bypass and/or the flow control valve 450 and the low-temperature SCR 430 as described in FIG. 1 b. In the event that the temperature of the exhaust gas flow is below the temperature threshold value, the exhaust gas flow is conducted through the low-temperature SCR and, immediately before this introduction, the low-temperature SCR is supplied with reductant.

FIG. 5 schematically shows a further advantageous embodiment of the present invention, which is illustrated by way of example as a supplement to FIG. 1 a. An SCR catalyst 520 for normal to high temperatures is located downstream of the engine (not shown) in FIG. 5. A reductant supply system 560, which comprises a reductant source, a reductant pump, and reductant dispenser or a reductant injector (not shown), is located directly upstream of such SCR catalyst 520. An exhaust gas temperature sensor 540 is arranged downstream of the SCR 520. An exhaust gas bypass and/or flow control valve 550, which communicates with the SCR 520 in such a way that it receives exhaust gas, is located downstream of the exhaust gas temperature sensor 540. The exhaust gas bypass and/or flow control valve 550 conducts the exhaust gas flow past the low-temperature SCR 530 via the bypass 580, if the exhaust gas flow has a temperature greater than or equal to a temperature threshold value. However, if the temperature of the exhaust gas flow is below this temperature threshold value, the exhaust gas flow is conducted through the low-temperature SCR 530, see arrow 570. An ASC 525 is located downstream of the low-temperature SCR 520.

FIG. 6a shows an exhaust gas bypass or flow valve, hereinafter referred to as a “bypass valve,” which is 100% open. In this case, “100% open” means that the exhaust gas paths 1 and 2 are connected so that exhaust gas from the HT-SCR flows into the TT-SCR. The arrow 1 shows the gas discharge from the HT-SCR and the entry into the bypass valve. Arrow 2 shows the discharge of the exhaust gas from the bypass valve toward the TT-SCR. Arrow 3 shows the discharge of the exhaust gas from the bypass valve toward the bypass. In FIG. 6a , the outlet 3 is completely closed.

The valve position according to FIG. 6a corresponds, as explained above, to a 100% open bypass valve. In this case, the following applies:

T_(actual)≤T_(min)<T_(Thd)

T_(actual)≥R_(min)<T_(Thd)

In this case.

-   -   T_(actual)=actual temperature of the HT-SCR     -   T_(min)=minimum operating temperature of the HT-SCR     -   T_(Thd)=temperature threshold value

As long as the actual temperature, i.e., the factual temperature of the HT-SCR, is below the temperature threshold value, the exhaust gas is conducted through the TT-SCR after exiting the HT-SCR, regardless of whether or not the minimum operating temperature of the HT-SCR has already been reached.

FIG. 6b shows a bypass valve, which is 100% closed. “100% closed” is equivalent to “0% open.” Here, the exhaust gas paths 1 and 3 are connected so that exhaust gas coming from the HT-SCR flows into the bypass and not through the TT-SCR. In this case, the following applies:

T_(actual)>T_(min)≥T_(Thd)

As soon as the temperature of the HT-SCR is greater than its minimum operating temperature and is greater than or equal to the temperature threshold value, the bypass valve is closed. After exiting the HT-SCR, the exhaust gas flow is then conducted past the TT-SCR.

If a flow valve or a combination of exhaust gas bypass and flow valve is exclusively used, conditions between 0% and 100% opening of the flow valve can be described by the condition

T_(actual)>T_(min)<T_(Thd)

In this condition, the actual temperature of the HT-SCR is greater than its minimum operating temperature but less than the temperature threshold value. In this case, a proportionality factor which indicates the degree of opening of the flow valve can be determined from (T_(Thd)−T_(actual)). In this case, the following boundary condition applies:

If

T _(actual) ≥T _(min) and T _(Thd) −T _(actual)≤0,

the bypass valve is 0% open, which is equivalent to 100% closed.

FIG. 7 shows the nitrogen oxide conversion values U_(Nox) [%] of three HT-SCR catalysts in the temperature range between 200 and 550° C. HT-SCR1 is a vanadium-containing SCR, HT-SCR2 is an iron-containing SCR and HT-SCR3 is a copper-containing SCR. The experimental conditions for the measurement of the nitrogen oxide conversion values are given in Exemplary Embodiment 4.

FIG. 8 shows the SO₂ curve after flowing through the DPF for three stationary sulfurization experiments on an engine test bench for a Cu zeolite HT-SCR. Such sulfurization experiments are described in Exemplary Embodiment 5. In this case, the SCR inlet temperature and space velocity in the SCR were varied for the three experiments.

FIG. 9 shows the SO₂ curve after flowing through the Cu zeolite HT-SCR for three stationary sulfurization experiments on an engine test bench. Such sulfurization experiments are described in Exemplary Embodiment 5. In this case, the SCR inlet temperature and space velocity in the SCR were varied for the three experiments. Together with FIG. 8. This shows the ability of the Cu zeolite to store SO_(x) at different temperatures and space velocities.

At temperatures below the HT-SCR minimum temperature and thus also below the threshold value (EOP_01), no SO₂ breakthrough can be observed for the test duration, which means good protection for the TT-SCR.

By increasing the temperature and/or the space velocity, the SO_(x) trap efficiency decreases somewhat. However, since in this case the NO_(x) conversion can be ˜100% (see FIGS. 7 and 10), the TT-SCR can already be switched into the bypass in these cases.

FIG. 10 shows the NO_(x) conversion of the Cu zeolite for the three sulfurization experiments. As can also be derived from FIG. 7, the HT-SCR shows no full conversion at 190° C. since such temperature is below its minimum temperature. Nevertheless, the conversion of 80% is maintained for a long time despite sulfur exposure. It can also be derived from FIG. 7 that at the beginning of the sulfurization, ˜100% NO_(x) conversion arises at EOP_02 and EOP_03.

FIG. 11 shows the sulfur breakdown curve from dynamic engine test bench experiments for a V HT-SCR and a Cu HT-SCR. The V HT-SCR shows hardly any sulfur storage functionality for the test time. However, the Cu HT-SCR exhibits the property as sulfur trap even under dynamic conditions.

FIG. 12 shows the NO_(x) conversion curves from the dynamic engine test bench tests for a V HT-SCR and a Cu HT-SCR. Both systems show that they still retain their HT-SCR function even with sulfur exposure.

EMBODIMENTS Embodiment 1: HT-SCR1=V SCR

-   a) A commercially available titanium dioxide in anatase form doped     with 5% by weight silica was dispersed in water, and vanadium     dioxide (VO₂) and tungsten trioxide (WO₃) were subsequently added to     such an amount that a catalyst of the composition 88.10% by weight     TiO₂, 4.60% by weight SiO₂, 3.00% by weight V₂O₅, 4.30% by weight     WO₃ results. The mixture was stirred thoroughly and finally milled     in a commercially available agitator bead mill. -   b) The dispersion obtained according to a) was coated onto a     commercially available ceramic flow-through substrate with a volume     of 0.5 l and a cell number of 62 cells per square centimeter at a     wall thickness of 0.17 mm over its entire length with a washcoat     loading of 160 g/l. Subsequently, drying took place at 90° C., and     calcination took place at 600° C. for 2 hours. The catalyst obtained     in this way is referred to below as HT-SCR1.

Embodiment 2: HT-SCR2=Fe SCR

-   a) A commercially available BEA-type zeolite with a SAR of 25 is     mixed in water with a quantity of Fe(NO₃)₃ corresponding to an iron     content of 4.5% by weight (based on the iron-containing zeolite and     calculated as Fe₂O₃) and stirred overnight. -   b) The dispersion obtained according to a) was coated onto a     commercially available ceramic flow-through substrate with a volume     of 0.5 l and a cell number of 62 cells per square centimeter at a     wall thickness of 0.17 mm over its entire length with a washcoat     loading of 220 g/l. The catalyst obtained (hereinafter referred to     as HT-SCR2) is dried at 90° C., then calcined step-by-step in air at     350° C. and at 550° C.

Embodiment 3: HT-SCR3=Cu SCR

-   a) A commercially available CHA-type zeolite with a SAR of 30 is     mixed in water with a quantity of CuSO₄ corresponding to a copper     content of 3.7% by weight (based on the copper-containing zeolite     and calculated as CuO) and stirred overnight. -   b) The dispersion obtained according to a) was coated onto a     commercially available ceramic flow-through substrate with a volume     of 0.5 l and a cell number of 62 cells per square centimeter at a     wall thickness of 0.17 mm over its entire length with a washcoat     loading of 220 g/l. The catalyst obtained (hereinafter referred to     as HT-SCR3) is dried at 90° C., then calcined step-by-step in air at     350° C. and at 550° C.

Embodiment 4: Determination of DeNOx Activity:

-   b) The DeNOx activity of the catalysts HT-SCR1 to HT-SCR3 was tested     in a laboratory model gas system under the conditions given in the     table below.

Gas/parameter Concentration/conditions NH₃ 1100 ppm NO 1000 ppm H₂O 5% O₂ 10% N2 Remainder Temperature Cooling step-by-step 550 to 200° C. Space velocity 60.000 h⁻¹

During the measurement, the nitrogen oxide concentrations of the model gas were detected after flowing through the HT-SCR catalyst by means of FTIR (Fourier transform infrared spectrometry). The nitrogen oxide conversion, based on the ratio of NH₃ to NO, over the catalyst for each temperature measuring point was calculated as follows from the known metered nitrogen oxide contents, which were verified during conditioning at the beginning of the respective test run with a pre-catalyst exhaust gas analysis, and the measured post-catalyst nitrogen oxide contents.

${U_{NO_{x}}\lbrack\%\rbrack} = {\left( {1 - \frac{c_{Output}\left( {NO}_{x} \right)}{c_{Input}\left( {NO}_{x} \right)}} \right) \times 100}$

where

U_(NOx) nitrogen oxide conversion

C_(Output) concentration of the nitrogen oxides after flowing through the HT-SCR catalyst

C_(Input) concentration of the nitrogen oxides before flowing through the HT-SCR catalyst

C _(Input/Output)(NO_(x))=C _(Input/Output)(NO)+C _(Input/Output)(NO₂)+C _(Input/Output)(N₂O)

The nitrogen oxide conversion values U_(NOx) [%] obtained were applied as a function of the pre-catalyst temperature measured to evaluate the SCR activity of the materials investigated. This is shown in FIG. 7.

TABLE 1 Nitrogen oxide conversion values of the three HT-SCR catalysts according to Exemplary Embodiments 1 to 3 U_(NOx) [%] T [° C.] HT-SCR1 HT-SCR2 HT-SCR3 200 26.13 28.36 85.71 250 71.79 62.29 99.41 300 95.17 85.48 100.07 350 98.58 94.55 100.07 400 98.87 97.98 99.76 450 97.76 99.29 96.79 500 92.03 99.44 90.65 550 69.13 96.20 83.03

Embodiment 5: Analysis of Cu SCR: SO_(x) Trap Function and HT-SCR:

The investigations of the HT-SCR3 with regard to its suitability as SCR catalyst for the medium to high temperature range and at the same time its suitability as SO_(x) trap in the working range of the TT-SCR were carried out on the engine test bench. The catalyst volume used corresponded to the factor ˜2.26 of the engine cubic capacity or displacement. Customary analysis was used as analysis. For the NO_(x) conversion determination, the pre-SCR and post-SCR NO_(x) concentrations were measured by means of CLDs (chemiluminescence detectors). The conversion was calculated analogously to Example 4.

${U_{NO_{x}}\lbrack\%\rbrack} = {\left( {1 - \frac{c_{Output}\left( {NO}_{x} \right)}{c_{Input}\left( {NO}_{x} \right)}} \right) \times 100}$

where

U_(NOx) nitrogen oxide conversion

C_(Output) concentration of the nitrogen oxides after flowing through the HT-SCR catalyst

C_(Input) concentration of the nitrogen oxides before flowing through the HT-SCR catalyst

The SO_(x) input and output concentrations were measured as SO₂ by a mass spectrometer each at the DPF output and SCR output. The experiments were carried out in stationary operating mode. For this purpose, three different operating points were selected which differ with regard to their SCR inlet temperature and the exhaust gas mass flow or the SCR space velocity (GHSV or SV).

GHS stands for “gas hourly space velocity” and SV stands for “space velocity.”

Commercially available urea solution was used as reductant. An amount corresponding to an ammonia/NOX ratio of 1.2 was metered.

Table 2 compiles the points. Here, EOP stands for engine operating point.

TABLE 2 SCR inlet temperature and SCR space velocity for three EOPs examined T SCR in [° C.] SV [h⁻¹] EOP_01 190 7700 EOP_02 260 23100 EOP_03 260 34000

EOP_01 represents an operating point at which the exhaust gas temperature is below the threshold value and at the same time below the minimum temperature of the HT-SCR.

Wth regard to their temperature, EOP_02 and EOP_03 are above the minimum temperature.

For the sulfurization, B10 diesel fuel was used, which was additionally mixed with sulfur. Fuels of this type are commercially available. For all three operating points, the experiment was carried out until ˜2 g of sulfur (not SOx but S) per liter of SCR volume was emitted.

The SO2 curve after passage through the DPF is shown in FIG. 8 and the SO2 curve after passage through the SCR is shown in FIG. 9.

Embodiment 6: NOx Conversion and SO₂ Trap Function of Cu HT-SCR and V HT-SCR

The comparison of a V SCR with a Cu SCR with regard to its suitability as a combined HT-SCR for the medium to high temperature range and as SO_(x) trap in the working range of the TT-SCR was carried out on the engine test bench. The vanadium-containing V SCR (HT-SCR1) and the copper-containing Cu SCR (HT-SCR3) according to Exemplary Embodiments 1 and 3 were tested. The catalyst volume used corresponded to the factor ˜1.1 of the engine cubic capacity or displacement. For the NO_(x) conversion determination, the pre-SCR and post-SCR NO_(x) concentrations were measured by means of CLDs (chemiluminescence detectors). The conversion was calculated as in Example 5.

The SO_(x) input and output concentrations were measured as SO₂ by a mass spectrometer each at the DPF output and SCR output. The experiments were carried out in dynamic operating mode. To this end, a sequence of WHTC cycles (world harmonized transient cycles) was performed. WHTCs are legally predefined test cycles used for testing, qualifying and releasing heavy-duty/payload engines. The performance and design of a WHTC and the implementation of the test procedure are known to the person skilled in the art.

Commercially available urea solution was used as reductant. An amount corresponding to an ammonia/NO_(x) ratio of 0.8 was metered. This is substoichiometric and the HT-SCR therefore cannot achieve 100% conversion. The theoretical maximum conversion would accordingly correspond to 80%.

For the sulfurization, B10 diesel fuel was used, which was additionally mixed with sulfur. Fuels of this type are commercially available. The sulfur emission per WHTC was ˜1375 mg.

The NO_(x) conversion of the Cu HT-SCR is shown in FIG. 10.

The sulfur breakdown curve from the dynamic engine test bench experiments for a V HT-SCR and the Cu HT-SCR is shown in FIG. 11.

FIG. 12 shows the NO_(x) conversion curves from the dynamic engine test bench tests for a V HT-SCR and a Cu HT-SCR. In this case, the cumulative mass of SO₂ per cycle, measured at the output of the SCR, is plotted against the number of sulfation cycles. 

1. An exhaust gas aftertreatment system coupled to an internal combustion engine in such a way that it receives the exhaust gas flow, comprising a selective catalytic reduction catalyst for medium to high temperature ranges (HT-SCR), wherein the medium temperature range comprises temperatures of 250° C. to less than 450° C. and the high temperature range comprises temperatures of 450° C. to 750° C., which catalyst is designed both to reduce NOx in an exhaust gas having a temperature above a temperature threshold value, and to store the SOx contained in the exhaust gas across the temperature range of the exhaust gas, which is below a temperature threshold value, a reductant supply system arranged directly upstream of the HT-SCR, a low-temperature SCR (TT-SCR) arranged downstream of the HT-SCR, wherein the low temperature range comprises temperatures of 60° C. to less than 250° C., and wherein the TT-SCR is designed to reduce NOx in an exhaust gas having a temperature below a temperature threshold value, a temperature sensor arranged directly downstream of the HT-SCR and measuring the temperature of the exhaust gas flow exiting such HT-SCR, an exhaust gas bypass and/or flow control valve designed to conduct the exhaust gas flow in its entirety past the TT-SCR if such exhaust gas flow has a temperature greater than or equal to a temperature threshold value, wherein the exhaust gas bypass and/or flow control valve is arranged directly upstream of the TT-SCR.
 2. The exhaust gas aftertreatment system according to claim 1, wherein the HT-SCR is present in the form of a catalytically active layer on a carrier substrate, wherein the catalytically active layer is a molecular sieve selected from an aluminosilicate having a SAR of 5-50 and a silicon aluminum phosphate (SAPO) having an (Al+P)/Si value of 4-15 is selected, wherein the molecular sieve contains 1-10% by weight of a transition metal selected from Fe, Cu, and mixtures thereof, calculated as Fe₂O₃ and CuO respectively, based on the total weight of the molecular sieve, and wherein the molecular sieve contains alkali metal and alkaline earth metal cations selected from Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, and mixtures thereof in a total amount of ≤1% by weight, calculated in the form of the pure metals and based on the total weight of the molecular sieve, and wherein the molecular sieve contains the metals Co, Mn, Cr, Zr and Ni in a total amount of ≤1% by weight, calculated in the form of the pure metals and based on the total weight of the molecular sieve.
 3. The exhaust gas aftertreatment system according to claim 2, wherein the molecular sieve is selected from small-pore zeolites having a maximum pore size of eight tetrahedral atoms and beta zeolite.
 4. The exhaust gas aftertreatment system according to claim 2, wherein the molecular sieve is an Fe BEA having a SAR of 10 to 35, an alkali metal content of 0 to 0.7% by weight, calculated as pure metals, and an Fe content of 3 to 9% by weight, calculated as Fe₂O₃, wherein the alkali metal and Fe contents are each based on the total weight of the zeolite.
 5. The exhaust gas aftertreatment system according to claim 2, wherein the molecular sieve is a Cu CHA having a SAR of 10 to 35, an alkali metal content of 0 to 0.7% by weight, calculated as pure metals, and a Cu content of 1 to 7% by weight, calculated as CuO, wherein the alkali metal and Cu contents are each based on the total weight of the zeolite.
 6. The exhaust gas aftertreatment system according to claim 1, wherein the TT-SCR is a monolithic flow-through substrate with a manganese-containing coating as catalytically active layer.
 7. The exhaust gas aftertreatment system according to claim 6, wherein the manganese-containing coating is a manganese-containing mixed oxide selected from mixed oxides of the general formula Mn_(a)Me_(1-a)O_(b), where Me is one or more elements from the group Fe, Co, Ni, Cu, Zr, Nb, Mo, W, Ag, Sn, Ce, Pr, La, Nd, Ti and Y and where a=0.02-0.98 and b=1.0-2.5, mixed oxides of the general formula Mn_(w)Ce_(x)Me_(1-w-x)O_(y), where Me is one or more elements from the group Fe, Co, Ni, Cu, Zr, Nb, Mo, W, Ag, Sn, Pr, La, Nd, Ti and Y and where w=0.02-0.98, x=0.02-0.98 and y=1.0-2.5, spinels of the general formula MnMe₂O₄ or MeMn₂O₄, where Me is Fe, Al, Cr, Co, Cu or Ti.
 8. The exhaust gas aftertreatment system according to claim 1, and further comprising a second reductant supply system arranged directly upstream of the TT-SCR.
 9. The exhaust gas aftertreatment system according to claim 1, wherein the reductant supply system contains a) a reductant source, b) a reductant pump, and c) a reductant dispenser or a reductant injector.
 10. The exhaust gas aftertreatment system according to claim 1, wherein an oxidation catalyst is located directly upstream of the reductant supply system located directly upstream of the HT-SCR.
 11. The exhaust gas aftertreatment system according to claim 9, wherein a second HT-SCR is located between the HT-SCR and the temperature sensor.
 12. The exhaust gas aftertreatment system according to claim 9, wherein a catalytically coated diesel particulate filter (CDPF) is located between the oxidation catalyst and the reductant supply system.
 13. The exhaust gas aftertreatment system according to claim 9, wherein one or more HT-SCRs are arranged upstream of the oxidation catalyst and the reductant supply system is located directly upstream of the the HT-SCR that is closest to the internal combustion engine.
 14. The exhaust gas aftertreatment system according to claim 1, wherein an ammonia slip catalyst is arranged downstream of the TT-SCR.
 15. A method for treating an exhaust gas flow, comprising: providing an exhaust gas aftertreatment system comprising an SCR catalyst for medium to high temperatures (HT-SCR), a low-temperature SCR (TT-SCR) arranged downstream of the SCR for medium to high temperatures, a temperature sensor arranged directly downstream of the HT-SCR and an exhaust gas bypass and/or flow control valve arranged directly upstream of the low-temperature SCR, wherein the SCR catalyst for medium to high temperatures, wherein the medium temperature range comprises temperatures of 250° C. to less than 450° C. and the high temperature range comprising temperatures of 450° C. to 750° C., is designed both a) to reduce NOx in an exhaust gas having a temperature above a temperature threshold value, and b) to store the SOx contained in the exhaust gas across the temperature range of the exhaust gas which is below a temperature threshold value, and the low-temperature SCR is designed to reduce NOx in an exhaust gas having a temperature below a temperature threshold value, wherein the low temperature range comprises temperatures of 60° C. to less than 250° C., and wherein the temperature sensor measures the temperature of the exhaust gas flow exiting the HT-SCR, and wherein the exhaust gas bypass and/or flow control valve is designed to conduct the exhaust gas flow in its entirety past the low-temperature SCR, if such exhaust gas flow has a temperature greater than or equal to a temperature threshold value, storing the SOx contained in the exhaust gas in the HT-SCR, reducing NOx in an exhaust gas flow with the low-temperature SCR catalyst, if a temperature of the exhaust gas flow is within a low temperature range; conducting the exhaust gas stream in its entirety past the low-temperature SCR, if such exhaust gas flow has a temperature greater than or equal to a temperature threshold value, and reducing NOx in an exhaust gas flow with the SCR catalyst for normal to high temperatures, if the temperature of the exhaust gas flow is within a normal to high temperature range. 